This invention relates to an electroluminescent display device having a plurality of electroluminescent display elements each comprising a portion of an organic electroluminescent layer common to all the display elements and first and second electrode layers arranged on opposing sides of the electroluminescent layer at the display element locations.
The display device may be a simple segmented display device, for example, for displaying simple icons. Alternatively, the device may comprise a matrix display device having an array of individually driveable display elements arranged in rows and columns. Such matrix display devices, which can be used for displaying information and video applications, may be of simple passive matrix form or of active matrix form in which each display element is controlled by an associated switch means, usually in the form of TFTs (thin film transistors). In a simple passive matrix display device, the electroluminescent (EL) layer is provided between crossing sets of row and column address conductors at their intersections thereby forming a row and column array of electroluminescent display elements. By virtue of the diode-like I-V characteristic of the electroluminescent display elements, each element is capable of providing both a display and a switching function enabling multiplexed drive operation. In an active matrix device the associated switch means is operable to supply a drive current to the display element so as to maintain its light output for a significantly longer period. Thus, for example, each display element circuit is loaded with an analogue (display data) drive signal once per field period in a respective row address period which drive signal is stored and is effective to maintain a required drive current through the display element for a field period until the row of display elements concerned is next addressed. Examples of active matrix electroluminescent display devices using thin film organic electroluminescent materials are described in EP-A-0717445.
The organic EL layers described in this particular reference are molecular organic material layers comprised of an organic hole injecting and transporting zone in contact with one of the electrodes (anode) and an electron injecting and transporting zone forming a junction with the hole injecting and transporting zone. The hole injecting and transporting zone can be of a single material or multiple materials and comprises a hole injecting layer in contact with the electrode and a contiguous hole transporting layer interposed between the hole injecting layer and the electron injecting and transporting zone. The electron injecting and transporting zone can similarly be formed of a single material or multiple materials, and comprises an electron injecting layer in contact with the other (cathode) electrode and a contiguous electron transporting layer interposed between the electron injecting layer and the hole injecting and transporting zone. Recombination of the holes and electrons, and luminescence, occurs within the electron injecting and transporting zone adjacent the junction. The layers are normally vapour deposited. The anode electrodes are formed of ITO which, being transparent, allows generated light to pass therethrough and has a suitably high work function. The anode electrodes in the array are provided as discrete pads arranged regularly in rows and columns and the electroluminescent layer extends as a continuous layer over the array of anodes. Overlying this layer, a continuous layer of low work function material such as calcium or a magnesium silver alloy is provided to form an electrode layer common to all display elements, portions of this layer immediately over the display element anode electrodes constituting the cathode electrodes.
The composition of the EL layer can, however, vary and it is known to use EL layers which for example, do not comprise specific hole injecting and electron injecting regions but rely instead solely on the electrodes for this purpose. The term EL layer used herein is intended to include these types as well.
More recently, light emitting polymers (LEPS) have been utilised for the electroluminescent layer. An example of an active matrix organic electroluminescent display device using LEP material is described in the paper by T. Shimoda et al entitled xe2x80x9cCurrent Status and Future of Light-Emitting Polymer Display Driven by Poly-Si TFTxe2x80x9d in SID 99 Digest, pages 372-375. The structure of an electroluminescent display element described therein comprises a layer of PPV (poly (p-phenylenevinylene)), an ITO anode electrode, a PEDOT-PSS (polyethylene dioxytiophene-polystyrene sulphonate) layer disposed between the PPV layer and the ITO mode, and a cathode electrode layer comprising Alxe2x80x94Li on the other side of the PPV layer. The PEPOT-PSS material used for the hole transport layer is said to enhance efficiency by an order of magnitude. As in the above-described device, the display element anodes are provided as discrete pad electrodes and the electroluminescent layer extends as a continuous layer over the entire area of the array. Similarly the cathode layer is provided as a continuous layer common to all display elements. An example of a passive matrix display device using LEP material is described in WO96/36959.
Problems can be experienced in operation of these kinds of electroluminescent display devices in the form of undesirable cross-talk effects whereby the driving of one display element can affect the operation of neighbouring display elements. For example, when one display element is turned on to emit light then neighbouring display elements which are supposed to be off may be seen to be partially emitting.
It is an object of the present invention to provide an improved electroluminescent display device in which such unwanted cross-talk effects are reduced.
According to the present invention, an electroluminescent display device of the kind described in the opening paragraph is characterised in that between the first electrodes of adjacent display elements an electrical conductor is provided in contact with the same side of the electroluminescent layer as the first electrodes which is held at a predetermined potential so as to sink electrical currents which flow laterally in the electroluminescent layer. The invention stems from a recognition that the organic electroluminescent layer, or at least components thereof, particularly the charge injecting/transporting regions when present, can be slightly conductive, and that electrical currents may flow laterally in this layer between the respective first electrodes of adjacent display elements in certain circumstances which lead to unwanted cross-talk effects between the adjacent display elements. The electrical conductor provided between the adjacent electrodes, and serving as a current sink, prevents electrical current flowing in this manner between adjacent display elements.
The electrical conductor can conveniently be formed at the same time as the first electrodes of the display elements by appropriately patterning a deposited conductive layer used for the first electrodes.
In the case of an active matrix electroluminescent display device in which the display element first electrodes comprise respective individual pad electrodes, the electrical conductor is preferably arranged to extend around the periphery of the first electrode, in the manner of a guard ring for example, to prevent current flowing laterally in the EL layer between adjacent display element first electrodes in both the row direction and the column direction. The electrical conductors required for the array of first electrodes may conveniently be provided in the form of a grid of electrically conductive material extending, preferably completely, around the individual pad electrodes. This grid can easily be formed at the same time as the pad electrodes by photolithographic patterning of a single deposited conductive layer used to form the pad electrodes, for example of ITO where the pad electrodes comprise the anodes. The continuous EL layer subsequently provided then extends over, and in contact with, both the pad electrodes and the grid, and with the hole transporting sub-layer component, if present, directly contacting the grid as well as the pad electrodes.
In the case of a passive matrix electroluminescent display devices, comprising crossing sets of parallel conductor strips, constituting respectively first and second electrodes of the display elements, with the EL layer extending there between, then the electrical conductor preferably comprises part of a conductive line extending between an adjacent pair of conductor strips of the set providing the display element first electrodes. Thus, where for example the conductor strips comprise the column conductors, a single conductive line serves to provide the electrical conductors between the first electrodes in two adjacent columns of display elements. Again, the required conductive lines can conveniently be provided at the same time as the first electrodes, i.e the one set of conductor strips, by appropriately patterning photolithographically a deposited conductive layer used to provide that set of conductor strips, and may for example comprise ITO where the set of conductor strips comprise the display element anode electrodes. A similar approach can be used to provide conductive lines between adjacent pairs of the other set of conductor strips as well.